In a nutshell, for categories too big for a single group of judges, the flight is split amongst teams of judges. When done, they come back together and re-taste the top beers from each group. From that tasting they choose 1,2,3.

Here's how that impacts the 2 v. 3 bottle rule: In 2 bottle competitions, the organizers have to ensure that the judges save enough beer for a mini-BOS and that the bottles are immediately re-capped and kept cold. It doesn't always help and the beers suffer before the mini. (Oh, the number of times I've heard "This really was fantastic when it was freshly opened"). Requiring 3 bottles ensures that everyone is on even footing for the mini-BOS.

Also, say something is wrong with one bottle (e.g. poorly capped, no gas, infected, etc) - judges in a 3 bottle comp will request the additional bottle to check to make sure the problem isn't confined to the single bottle.

On the plus side for the 2 bottle rule: organizers need 1/3 less storage space for entries before the competition.